X-ray tube technology is well known and some embodiments include a vacuum tube configured with a cathode that emits electrons that collide with an anode “target” to produce x-ray emissions. Those of ordinary skill in the art appreciate that x-ray emissions are used for a variety of applications that typically include directing the x-rays at a sample and measuring a response. One particular application includes what is generally referred to as “High Resolution Imaging” that relies on a very small spot size to provide a high degree of resolution, preferably with a substantial degree of power. For example high resolution imaging with good off-axis detection is an important technology in the semiconductor manufacturing industry where highly accurate 3-dimensional images provide various benefits such as quality control for the manufacturing process.
One particular x-ray tube configuration includes what is referred to as an “end-window” configuration that directs x-ray emissions through a planar window towards the surface of the sample. The end-window type of tubes can further be divided into two classes such as what are referred to as “transmission” based tubes and “reflection” based tubes. For example, embodiments of transmission type tubes typically position the anode in the path of an electron beam directed to the sample from the cathode. In this way the electron beam passes through the anode which produces x-ray emissions that interact with the sample. This type of configuration produces excessive amounts of heat that limits the amount of power that can be used. Further, passing the electron beam through the anode attenuates the power of the electrons as they pass through (e.g. amount of attenuation depends, at least in part, upon the thickness of the anode material).
Alternatively, reflection type tubes have an anode positioned within the vacuum chamber, where the anode comprises a target face (also referred to as a reflection target, a reflective surface, a target face, or a target surface, each of which may be used interchangeably herein) in proximity to an electron gun type cathode. The electron gun emits an electron beam that interacts with the target surface to produce x-ray emissions that are “reflected” through the window. Reflection type tubes typically produce less heat than a transmission type tube thus enabling use of higher power as well as a higher efficiency of power delivery to the sample (e.g. because there is substantially no attenuation from the anode material).
Importantly, the target anode of reflection type x-ray tube configurations have a number of technical aspects that must be considered for reliable operation. First, the target anode must accurately intersect with an electron beam to allow the creation of characteristic x-rays that are directed towards an output window. A second consideration is that the target anode should be maintained at a high voltage potential that is substantially equal to the operating voltage of the tube while being positioned in close proximity to structures that should be maintained at ground potential (e.g. the electron gun, outer cylinder, etc.). As those skilled in the art appreciate, large differences in voltage potential of objects located in close proximity to one another can often result in arcing problems.
A third consideration is that target anode structure must also efficiently carry heat away from the target surface where the electron beam is focused, which produces the x-ray emissions and heat. Again, it is appreciated that if the target anode configuration lacks sufficient heat transfer characteristics it may be inoperable, or at least require a reduced level of power that could be used in order to avoid overheating and possible damage to the target anode structure.
A fourth consideration is that the configuration of the target anode structure effects the characteristics of the electric field in the x-ray tube which in turn affects the electron trajectory and voltage standoff distance characteristics (e.g. standoff voltage distance includes a distance where arc-over between the cathode and the anode does not occur). It will further be appreciated that optimization of one consideration can have a negative impact on another consideration, and thus the relative impacts on performance should be contemplated. For example, the thermal transfer capacity of a target anode structure is limited by the cross-sectional area, where a large anode cross section carries more heat but can compromise voltage standoff distance from other objects such as an electron gun.
One target anode configuration that has been employed comprises a long rod with a beveled end to improve the spacing between the target surface of the anode facing the cathode and other structures maintained at ground potential. The beveled end can also be positioned in closer proximity to the window as compared to non-beveled embodiments due to a better voltage standoff distance. This is a very simple and cost effective solution, but has limitations as to the considerations described above. For example, with a straight rod structure there is a compromise between window proximity and electron gun proximity in order to maintain adequate voltage standoff distances to avoid arc-over.
Another possible target anode configuration includes a rectangular rod structure that is stepped down in the distal portion to allow for a relatively close proximity to the window similar to the beveled end embodiments described above. This helps to overcome the configuration problem of the straight rod, but makes the angles of intersection with the electron beam difficult. For example, the stepped down configuration lengthens the electron path between the electron gun cathode and the target anode surface making focusing the x-ray emissions a challenge. Additionally, the sharp transitions in cross-section create high voltage stress points that reduce the voltage standoff in the tube and affect the characteristics of the electric field.
Therefore, it is appreciated that a reflection type x-ray tube design that has a very small focal spot and large cone of illumination while simultaneously providing effective heat dissipation provides a substantial benefit for applications such as High Resolution Imaging.